In the preparation of high quality semiconductor material (e.g., silicon films) for use in the microelectronics industry, it is well known that contaminants must be controlled. Failure to control contaminants, as is also well known and appreciated, can result in the loss of significant resources as the resultant products are typically not useful for their intended purposes.
Generally, the starting materials in the fabrication of silicon films consist essentially of gases, typically denoted either "bulk" (e.g., nitrogen or argon) or "specialty" (e.g., hydrogen chloride, hydrogen bromide, boron trichloride). The successful operation of a fabrication facility designed to prepare semiconductor materials depends directly on the purity of the starting gases, as well as the gas handling capabilities available to preserve gas purity during the delivery of the gases to the process chamber and while material processing is taking place. Suitable control of the purity of such starting gases (i.e., monitoring and inhibiting high levels of contaminants as may be contained in the gases) is essential.
Under many current techniques, such control is achieved after the fact. That is, the silicon films so produced are periodically tested and the production line shut down only after such tests reveal the presence of high level contaminants. These processes, as will be appreciated by those skilled in the art, can lead to the waste of not only starting materials but also product which is produced prior to cessation of production. It is therefore desirable to monitor and control the contaminants as may be contained in such starting gases during production so as unacceptable contaminant levels are observed, production can be immediately, or at least shortly thereafter, halted.
Many molecular, atomic, radical, and ionic species are present in the bulk and specialty gases used in the preparation (e.g., chemical vapor deposition or "CVD") and processing (doping and etching) of semiconductor materials that can be viewed as "contaminants." Such contaminants can degrade either the quality of the fabricated semiconductor material or the efficiency with which the semiconductor material is prepared. These contaminant species can interfere with the chemical process directly or even cause particles to be formed in the gas delivery lines or process chamber, which subsequently deposit on the surface of the wafer material causing indirect performance defects.
The first step in controlling and/or eliminating these contaminants is their detection in the bulk and specialty gases used as starting materials. While this is generally recognized, heretofore practiced methods are generally inadequate. This is due, in large part, to the situation created by seemingly ever increasing competitive industry standards which have developed. Specifically, as the size of microelectronic devices has decreased while performance specifications have been intensified, the requirements for gas purity (i.e., absence of microcontamination) has increased.
Against this backdrop, it will likely be clear that several measurement criteria are important to detector effectiveness: (1) absolute detection sensitivity usually stated as parts-per-total number of gas molecules in the sample (e.g., parts-per-million or number of contaminant molecules per 10.sup.+6 background molecules); (2) species selectivity or the capability to measure the concentration of one species in the presence of other species; (3) rapidity of measurements to obtain a desired signal to noise ratio; (4) capability of monitoring contaminants in both non-reactive and reactive gases; and (5) linearity and dynamic range of gas concentrations that can be measured.
The current state-of-the-art devices for contaminant detection (e.g., water) encompass a variety of measurement techniques. For example, current state-of-the-art devices for water vapor detection utilize conductivity and electrochemical, direct absorption spectroscopy, and atmospheric pressure ionization mass spectroscopy (APIMS) measurement techniques. As discussed below, each of these methods fails to adequately address these requirements.
Conductivity and Electrochemical
Conductivity and electrochemical methods by solid-state devices exist which can detect water vapor at the 1 to 100 ppm range. Conductivity and electrochemical methods generally require direct physical contact between the sample and the device; thus, detection occurs after water molecules deposit on the solid-state surface. As a consequence, these devices do not perform well, if at all, with utilization of reactive or corrosive gases. Indeed, even their performance in non-reactive gases changes and/or deteriorates after even short exposures to reactive or corrosive gases. The linearity and dynamic range of response are usually limited to about one decade. The detection selectivity of these devices with respect to different gaseous species also is generally poor since the devices themselves will respond to a wide range of species without discrimination. Additionally, selectivity is incorporated into the measurements only through whatever chemical selectivity, if any, is embodied in the coatings used to cover these devices.
Direct Absorption
Direct absorption spectroscopy generally relates to the passing of light through the sample from an external source and measuring the reduction in light intensity caused by molecular, atomic, radical, and/or ionic absorption in the sample. Detection sensitivity depends directly on the subtraction of two large numbers (light intensity from the external source before it passes through the sample and its intensity after it exits the sample). This limits the detection sensitivity to the extent that direct absorption is generally considered a low sensitivity methodology.
APIMS
APIMS, initially used in the analysis of impurities in bulk nitrogen and argon and ambient air for air pollution studies, is now currently used by semiconductor manufacturers to detect trace levels of moisture and oxygen in inert bulk gases. With APIMS, the sampled gas is bombarded with electrons, or may be flame and photon excited, to produce a variety of ions that are then detected directly. Particularly, ionization occurs at atmospheric pressure in the presence of a reagent gas in the ionization source. APIMS typically exhibits detection sensitivities in the range of about 10 parts per trillion (ppt) in non-reactive gases. APIMS cannot even be used with reactive gas mixtures. Additional disadvantages of APIMS include an average cost between about $150,000 to $250,000, extensive purging and calibration procedures, and the need for a knowledgeable operator.
Intracavity Laser Spectroscopy
In the context of the present invention, laser technology, specifically intracavity laser spectroscopy (ILS), is disclosed as being used as a detector (sensor) to detect gaseous species (contaminants) at very high sensitivity levels. While the methods and apparatus disclosed herein are particularly suited for application in fabrication of semiconductor components, it should be appreciated that the present invention in its broadest form is not so limited. Nevertheless, for convenience of reference and description of preferred exemplary embodiments, this application will be used as a benchmark. In connection with this application, laser technology offers distinct advantages to gaseous species (contaminant) detection over known methods and, particularly, to water vapor detection.
In conventional applications of lasers to the detection of gaseous species (contaminants), laser produced radiation is used to excite the gas sample external to the laser in order to produce a secondary signal (e.g., ionization or fluorescence). Alternatively, the intensity of the laser after it passes through a gas sample, normalized to its initial intensity, can be measured (i.e., absorption).
Some twenty years ago, another detection methodology, intracavity laser spectroscopy, was first explored in which the laser itself is used as a detector; see, e.g., G. Atkinson, A. Laufer, M. Kurylo, "Detection of Free Radicals by an Intracavity Dye Laser Technique," 59 Journal Of Chemical Physics, Jul. 1, 1973.
Intracavity laser spectroscopy (ILS) combines the advantages of conventional absorption spectroscopy with the high detection sensitivity normally associated with other laser techniques such as laser-induced fluorescence (LIF) and multiphoton ionization (MPI) spectroscopy. ILS is based on the intracavity losses associated with absorption in gaseous species (e.g., atoms, molecules, radicals, or ions) found within the optical resonator cavity of a multimode, homogeneously broadened laser. These intracavity absorption losses compete via the normal mode dynamics of a multimode laser with the gain generated in the laser medium. Traditionally, ILS research has been dominated by the use of dye lasers because their multimode properties fulfill the conditions required for effective mode competition and their wide tunability provides spectral access to many different gaseous species. In particular, measurements at visible wavelengths have been conducted using dye lasers having linear two-mirror cavities; see, e.g., V. M. Baev, J. Eschner, J. Sierks, A. Weiler, and P. E. Toschek, "Regular dynamics of a multimode dye laser", Optics Communications, 94 (1992) 436-444; and J. Sierks, V. M. Baev, and P. E. Toschek, "Enhancement of the sensitivity of a multi-mode dye laser to intracavity absorption", Optics Communications, 96 (1993) 81-86.
The liquid dye laser, however, is not compatible with many practical applications given its liquid state and the need to maintain physical and optical stability. Dye lasers also operate primarily in the visible spectral region. The absorption strength of any gaseous species, although definitely detectable by ILS, are not as strong in the visible as compared to lower energies (e.g., in the near infrared). Higher detection sensitivity, therefore, is found when absorption transitions in the infrared are utilized.
Some ILS experiments have been performed with multimode, tunable solid-state laser media such as color centers and Ti:Sapphire; see, e.g., D. Gilmore, P. Cvijin, G. Atkinson, "Intracavity Absorption Spectroscopy With a Titanium: Sapphire Laser," Optics Communications 77 (1990) 385-89.
ILS has also been successfully used to detect both stable and transient species under experimental conditions where the need for high detection sensitivity had previously excluded absorption spectroscopy as a method of choice. For example, ILS has been utilized to examine gaseous samples in environments such as cryogenically cooled chambers, plasma discharges, photolytic and pyrolytic decompositions, and supersonic jet expansions. ILS has been further used to obtain quantitative absorption information (e.g., line strengths and collisional broadening coefficients) through the analysis of absorption lineshapes. Some of these are described in G. Atkinson, "Intracavity Laser Spectroscopy," SPIE Conf., Soc. Opt. Eng. 1637 (1992).
Prior art methods of performing ILS, however, while suitable for use in laboratory settings are unacceptable for commercial settings. The constraints of commercial reality, as briefly noted above, essentially dictate that such a detector be conveniently sized, relatively inexpensive, and reliable. Laboratory models fail to fully meet these requirements.
A laboratory demonstration of the feasibility of using ILS techniques for detecting small quantities of water vapor in a nitrogen atmosphere with a Cr.sup.4+ :YAG laser is described in D. Gilmore, P. Cvijin, G. Atkinson, "Intracavity Laser Spectroscopy in the 1.38-1.55 .mu.m Spectral Region Using a Multimode Cr.sup.4+ :YAG Laser," Optics Communications 103 (1993) 370-74. The experimental apparatus utilized was satisfactory for demonstration of operational characteristics, but undesirable for implementation in a commercial application as contemplated by the present invention.
In accordance with various aspects of the present invention, the present invention provides a user friendly, i.e., comparatively simple, detection system, having the advantages of direct absorption techniques but with dramatically increased detection sensitivities, capable of detecting gaseous species in reactive and non-reactive samples at a commercially viable cost. In this regard, the present invention addresses the long felt need for a method and apparatus for the high sensitivity detection of contaminants in reactive and non-reactive gas systems in commercial settings.